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Super Kai (Kazuya Ito)
Super Kai (Kazuya Ito)

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Shallow Copy & Deep Copy in Python (5)

#python #dictionary #shallowcopy #deepcopy

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*Memo:

  • My post explains the shallow and deep copy of a list.
  • My post explains the shallow and deep copy of a tuple.
  • My post explains the shallow copy of the set with a tuple.
  • My post explains the shallow and deep copy of the set with an iterator.
  • My post explains the shallow and deep copy of an iterator.
  • My post explains a dictionary(1).

<Normal Copy>:

*Memo:

  • v1 and v2 refer to the same shallow and deep dictionary.
  • is keyword can check if v1 and v2 refer to the same dictionary. 
 #### Shallow dictionary #### # ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓ ↓ v1 = {'k':'v_a', 'd':{'k':'v_b'}} v2 = v1 # ↑↑↑↑↑↑↑↑↑↑↑  # Deep dictionary  print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v1 is v2, v1['d'] is v2['d']) # True True  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' # ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'V_A', 'd': {'k': 'V_B'}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B'}}  # ↑↑↑↑↑ ↑↑↑↑↑
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<Shallow Copy>:

dict.copy() does shallow copy as shown below:

*Memo:

  • v1 and v2 refer to the different shallow dictionaries.
  • v1 and v2 refer to the same deep dictionary. 
v1 = {'k':'v_a', 'd':{'k':'v_b'}} v2 = v1.copy() print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b'}}  print(v1 is v2, v1['d'] is v2['d']) # False True  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' # ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'v_a', 'd': {'k': 'V_B'}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B'}}  # ↑↑↑↑↑ ↑↑↑↑↑
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The below with copy.copy() which does shallow copy is equivalent to the above:

import copy v1 = {'k':'v_a', 'd':{'k':'v_b'}} v2 = copy.copy(v1) print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b'}}  print(v1 is v2, v1['d'] is v2['d']) # False True  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' # ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'v_a', 'd': {'k': 'V_B'}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B'}}  # ↑↑↑↑↑ ↑↑↑↑↑
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The below with dict() which does shallow copy is equivalent to the above:

v1 = {'k':'v_a', 'd':{'k':'v_b'}} v2 = dict(v1) print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b'}}  print(v1 is v2, v1['d'] is v2['d']) # False True  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' # ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'v_a', 'd': {'k': 'V_B'}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B'}}  # ↑↑↑↑↑ ↑↑↑↑↑
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<Deep Copy>:

copy.deepcopy() does deep copy as shown below:

*Memo:

  • v1 and v2 refer to the different shallow and deep dictionaries.
  • copy.deepcopy() should be used because it's safe, doing copy deeply while dict.copy(), copy.copy() and dict() aren't safe, doing copy shallowly. 
import copy v1 = {'k':'v_a', 'd':{'k':'v_b'}} v2 = copy.deepcopy(v1) print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b'}}  print(v1 is v2, v1['d'] is v2['d']) # False False  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' # ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B'}}  # ↑↑↑↑↑ ↑↑↑↑↑
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The below with dict.copy() which does shallow copy is equivalent to the above:

v1 = {'k':'v_a', 'd':{'k':'v_b'}} v2 = v1.copy() v2['d'] = v1['d'].copy() print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b'}}  print(v1 is v2, v1['d'] is v2['d']) # False False  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' # ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B'}}  # ↑↑↑↑↑ ↑↑↑↑↑
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The below with copy.copy() which does shallow copy is equivalent to the above:

import copy v1 = {'k':'v_a', 'd':{'k':'v_b'}} v2 = copy(v1) v2['d'] = copy(v1['d']) print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b'}}  print(v1 is v2, v1['d'] is v2['d']) # False False  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' # ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B'}}  # ↑↑↑↑↑ ↑↑↑↑↑
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The below with dict() which does shallow copy is equivalent to the above:

v1 = {'k':'v_a', 'd':{'k':'v_b'}} v2 = dict(v1) v2['d'] = dict(v1['d']) print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b'}}  print(v1 is v2, v1['d'] is v2['d']) # False False  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' # ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'v_a', 'd': {'k': 'v_b'}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B'}}  # ↑↑↑↑↑ ↑↑↑↑↑
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Additionally, the below is a 3D dictionary:

import copy v1 = {'k':'v_a', 'd':{'k':'v_b', 'd':{'k':'v_c'}}} v2 = deepcopy(v1) print(v1) # {'k': 'v_a', 'd': {'k': 'v_b', 'd': {'k': 'v_c'}}} print(v2) # {'k': 'v_a', 'd': {'k': 'v_b', 'd': {'k': 'v_c'}}}  print(v1 is v2, v1['d'] is v2['d'], v1['d']['d'] is v2['d']['d']) # False False False  v2['k'] = 'V_A' v2['d']['k'] = 'V_B' v2['d']['d']['k'] = 'V_C' # ↓↓↓↓↓ ↓↓↓↓↓ ↓↓↓↓↓ print(v1) # {'k': 'v_a', 'd': {'k': 'v_b', 'd': {'k': 'v_c'}}} print(v2) # {'k': 'V_A', 'd': {'k': 'V_B', 'd': {'k': 'V_C'}}}  # ↑↑↑↑↑ ↑↑↑↑↑ ↑↑↑↑↑
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